The game will be the same regardless if its in multiplayer or single player, the difference is on the number of players on the "server". Once the game is released they will run an official server that people can join (the MMO-ish part), but private and single player servers will be possible as well (just like Freelancer).

You will be able to do Squadron 42 on the multiplayer servers too and have people join you in battle (it's instanced).

Currently they are focusing on Squadron 42, but combat multiplayer will be released first so they can balance it and get it tested.